IronPete, To plant bulbils or rounds outdoors in a container for the winter, the container needs to have really good drainage for starters. I know our climates are really different although we are geographically close, but I will say that I had containers I thought were well drained, but under certain conditions we did have a thaw and rain didn't run off because the soil was still frozen, then the water refroze as ice on the surface. This happened a couple years ago in some larger tubs I had planted, and the survivorship was really poor. The survival was better in a large but shallower flat, maybe because the side of it is broken so water runs off really easily, or maybe because there wasn't such a large volume of frozen soil, that it thawed sufficiently to drain? But even in this container the weather this past winter was too extreme - we had repeated heavy rain followed by freeze, then more heavy rain, repeat freeze, etc. We always get some of that but this time it was more severe.
The second thing that causes losses is extreme cold when the ground is not protected by heavy mulch or snow. Bulbils and rounds are more susceptible, maybe because they are smaller and they aren't planted as deep as a large clove. But even mature cloves can be killed if the cold is severe and not mulched or snow covered. Leaves or kelp make good mulches.
I am looking for new techniques or strategies to improve the survivorship of bulbils and rounds whether in containers or in the ground. In ground and raised bed is a safer place than a container, from a drainage standpoint. There may be other issues though, like predation by wireworms. I'm not sure why the survival of my last crop of rounds in the ground was only about 70%, whether the mulch was not good enough, wireworms et some, or what.
I might try hilling the soil over them, then mulch, and then smooth it out in spring so they're not too deep.
There has to be a better hedge against wierd winter weather.